1. Field of Invention
The invention relates to a bonding structure and, in particular, to a bonding structure of device packaging.
2. Related Art
With the increasing requirements of the high device reliability, high bonding density, and device size reduction in semiconductor device packaging technology, conventional wire bonding is gradually replaced by the flip-chip technology.
The flip-flop packaging technology is used to for pads or bumps at the junctions of a device and a substrate, in place of the lead frame used in the prior art, followed by coating a layer of an adhesive agent on the substrate surface. The structure is formed by directly embossing or welding the bumps of a device and the pads of a substrate together. In comparison with wire bonding, the prior art can reduce the transmission distance of electrical signals, which is suitable for the packaging of high-speed electronic devices. However, in the conventional flip-flop packaging method, the adhesive agent coated on the substrate has a serious difference in the coefficient of thermal expansion with respect to the device. When the temperature changes, the thermal stress is likely generate deformation at the bumps between the device and the substrate.
The adhesive agent used in normal flip-flop packaging can be divided into non-conductive films (NCF) and anisotropic conductive films (ACF). The conventional bonding technology coats NCF on a substrate and then bonds devices thereon by melting the NCF through pressing and heating procedures. The contraction stress produced after the film is completed cured bonds the devices together. The bonding technology can provide a higher bonding density. However, the bonding among the devices is maintained by a mechanical force. That is, the stress produced by the film maintains the conduction quality of the pads. Once the film experiences a too large stress, lamination will occur to the interfaces between the film and the circuit and substrate, increasing the resistance.
The ACF bonding technology places an ACF with conductive particles between a device and another device to be bonded. Pressing and heating procedures are employed to melt the film, bonding the devices together. A conductive channel is thus formed among metal pads, metal bumps, and conductive particles. The drawback of this technology, however, is that if the bonding pitch between adjacent metal bumps is very small, bridging phenomena will be happened. Due to the pressure and heat, conductive particles have drifting motions to result in short circuiting between adjacent two conductive points. Therefore, it cannot satisfy the requirement of miniaturization and the bonding density allows a pitch of 40 μm.
Another diffusion bonding technology makes use of high temperature to produce diffusion between the pads of devices and the substrate for bonding. However, the bonding temperature is often higher than 400 degrees of Celsius. The metal surfaces of the pads thus form metal oxide. Its covalence bond constrains free electrons of the metal, making it hard to form metal bonds at the interface. Moreover, the conduction is a result of the electron tunneling effect. There is a higher resistance at the connecting points. Therefore, it is not suitable for the applications in fine pitches.